Atherosclerosis is a complex, polygenic disease of arterial degeneration that can lead to coronary heart disease (CHD). In Western societies, complications arising from atherosclerosis are the most common causes of death. Several risk factors for CHD have been well-established, and include elevated low density lipoprotein-cholesterol (LDL-C), low levels of high density lipoprotein cholesterol (HDL-C), cigarette smoking, hypertension, age, and family history of early CHD. Given the abundant clinical data and epidemiological studies indicating that lowering LDL-C is beneficial for the prevention of adverse coronary events, the primary target of pharmacological intervention in the treatment and prevention of CHD is lowered LDL-C.
In individuals with autosomal dominant hypercholesterolemia (ADH), elevated LDL-C levels have been linked to mutations in the genes encoding LDL-receptor (LDL-R), apolipoprotein B (apoB), or proprotein convertase subtilisin/kexin type 9 (PCSK9) (Abifadel et al., Nat. Genet., 2003, 34:154-156). The PCSK9 gene (also known as FH3; NARC1; NARC-1; and HCHOLA3) was mapped to Chromosome 1 at location 1p32.3. PCSK9 was identified as a third locus associated with ADH when gain-of-function mutations in PCSK9 were found to be linked to elevated LDL-C levels. ApoB-100 participates in the intracellular assembly and secretion of triglyceride-rich lipoproteins and is a ligand for the LDL-R. PCSK9 is proposed to reduce LDL-R expression levels in the liver. Reduced LDL-R expression results in reduced hepatic uptake of circulating apoB-100-containing lipoproteins, which in turn leads to elevated cholesterol.
Familial hypercholesterolemia (FH) is caused by hundreds of different mutations in the LDL-R, and is phenotypically characterized by elevated plasma LDL-C levels and deposits of LDL-C in tendons, skin and arteries, leading to premature cardiovascular disease. Homozygous and heterozygous mutations in the LDL-R are associated with FH. Likewise, heterozygous and homozygous mutations in the ligand-binding domain of PCSK9 are associated with the FH phenotype. Mutations in this gene have been associated with a third form of autosomal dominant FH.
In mice genetically deficient in PCSK9, a marked increase in LDL-R expression and increased plasma LDL clearance rate were observed. Conversely, overexpression in mice of wild-type PCSK9, or certain PCSK9 mutants, promotes decreased LDL-R expression and elevated LDL-C levels.